Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1085
Mahmud b. Labid reported: When 'Uthman b. 'Affan intended to build the mosque (of the Prophet) the people did not approve of it. They liked that it should be kept in the same state. Thereupon he said: I heard the Messtnger of Allah (may peace be upon him) say: He who built a mosque for Allah, Allah would build a house for him like it in Paradise.